1. Field of the Invention
The present invention relates to a liquid crystal panel, a liquid crystal display device using this liquid crystal panel, and electronic equipment incorporating this liquid crystal display device.
2. Description of the Related Art
Liquid crystal display devices ranging from small-sized devices used in projectors, cellular telephones, and the like to large-sized devices used in laptop personal computers, monitors, televisions, and the like are spreading rapidly along with the development of the multimedia age. In the meantime, medium-sized liquid crystal display devices are indispensable for electronic equipment such as viewers or personal digital assistants (PDAs) and for amusement machines such as portable game players and pachinko (Japanese pinball machines). In addition, liquid crystal display devices are used in many other devices including home electric appliances such as refrigerators, microwave ovens, and the like.
Today, most of liquid crystal display elements adopt the twisted nematic (TN) display mode. The liquid crystal display elements of the TN display mode apply nematic liquid crystal compositions, and driving methods for the liquid crystal are roughly divided into two methods. One of the methods is a simple matrix driving method. The other one is an active matrix method in which each pixel is provided with a switching element such as a thin-film transistor (TFT). At present, the TN-TFT mode which is a combination of the TN display mode and the active matrix method using TFTs is generally applied, for example.
Apart from the TN display mode, another mode for the liquid crystal display elements is called a super twisted nematic (STN) mode. Although contrast and viewing angle dependency are improved in this STN mode as compared to the conventional simple matrix method using the TN mode, the STN mode is not suitable for motion image displays due to slow response speed. In addition, the STN mode has another disadvantage which is low display quality as compared to the active matrix method using the TFTS. Accordingly, the liquid crystal display devices adopting the TN-TFT mode are now predominant in the market.
In the meantime, to meet the demand for higher image quality, various methods for improving the viewing angle have been developed and put to practical use. As a result, TFT active matrix liquid crystal displays adopting any of the following three modes, namely, a mode applying a compensating film to the TN mode, the in-plane switching (IPS) mode, and the multi-domain vertical aligned (MVA) mode, are now becoming mainstream for high-performance liquid crystal displays.
These active matrix liquid crystal display devices normally apply positive and negative writing of an image signal at a cycle of 30 Hz. Accordingly, an image is rewritten at a cycle of 60 Hz and time for one field is approximately equal to 16.7 ms (milliseconds). In this case, total time of both positive and negative fields is called one frame, which is approximately equal to 33.3 ms. In this context, the response speed of the current liquid crystal is almost equivalent to this frame time at the shortest.
There are two major demands for the liquid crystal display devices, namely, higher definition of images and improvement in response speed for images. The demand for the improvement in response speed reflects growing opportunities for liquid crystal display screens to display not only conventional still images but also motion images. Among those motion images, images particularly associated with high speed image changes such as sports videos and computer graphics in games require higher response speed than the current frame time.
Meanwhile, concerning the higher definition, 100 ppi (pixels per inch) is the current mainstream definition. There are two methods for achieving higher definition. One of the methods is to increase processing accuracy and reduce sizes of pixel elements, and the other one is to switch a backlight for illuminating a display device among red, green, and blue by time division and achieve multicolor displays by each pixel element. The latter method is referred to as the field sequential (time division) method and is now under consideration for application to color liquid crystal display devices. In this method, it is not necessary to divide the pixels into three groups and to spatially arrange corresponding color filters. Therefore, this method is deemed capable of achieving three times higher definition than conventional display devices and of improving light use efficiency due to an increase in numerical apertures. On the other hand, a field sequential liquid crystal display device is required to display one color within one-third of the time for one field, which is approximately equal to 5 ms. Therefore, the liquid crystal applied thereto must have the response speed less than 5 ms. After all, the improvement in response speed is also essential for achieving higher definition of images.
Various techniques have been studied to meet requirements for a liquid crystal display device which can respond to the above-described high speed images, and techniques concerning high speed liquid crystal display modes have been developed to date. These techniques concerning high speed liquid crystal display modes are categorized into two major trends. One of the trends is a technique to increase response speed of twisted nematic liquid crystal. The other one is a technique to use different liquid crystal which offers high speed response.
In general, the response time of liquid crystal is given by the following two formulae (see “Liquid Crystal Dictionary”, edited by Liquid Crystal Section, The 142-nd Committee for Organic Materials for Information Science, Japan Society for the Promotion of Science, Baifukan Co., Ltd, pp. 24-25). Specifically, the following Formula 1 holds true for response at rise time when a voltage higher than a threshold voltage is applied to create an on-state. Meanwhile, the following Formula 2 holds true for response at decay time when the applied voltage higher than the threshold voltage is suddenly reduced to 0. In this case, d denotes a thickness of a liquid crystal layer, η denotes rotational viscosity, Δ∈ denotes dielectric anisotropy, V denotes the applied voltage, Vc denotes the threshold voltage, and K denotes a Frank elastic constant.
                              τ          rise                =                                            d              2                        ×            η                                Δ            ⁢                                                  ⁢                          ɛ              ⁡                              (                                                      V                    2                                    -                                      V                    c                    2                                                  )                                                                        (                  Formula          ⁢                                          ⁢          1                )                                          τ          dcay                =                                            d              2                        ×            η                                              π              2                        ×            K                                              (                  Formula          ⁢                                          ⁢          2                )            
In the meantime, the following Formula 3 holds true for the TN mode. In this case, K11 denotes a splay elastic constant, K22 denotes a twist elastic constant, and K33 denotes a bend elastic constant.
                    K        =                              K            11                    +                                    1              4                        ⁢                          (                                                K                  33                                -                                  2                  ⁢                                      K                    22                                                              )                                                          (                  Formula          ⁢                                          ⁢          3                )            
As it is apparent from the Formula 1, in terms of the response at rise time, the response time of liquid crystal is proportional to the reciprocal of the squared size of the applied voltage. In other words, the response time of liquid crystal depends on the squared reciprocals corresponding to voltage values which vary from one tone level to another. For this reason, the response time varies largely depending on the tone level. For example, a 10 times voltage differential causes a 100 times differential in the response time. On the contrary, whereas the differential in the response time depending on the tone level also exists in the response at decay time, the differential falls in the range of about 2 times.
From these aspects, the speed of the response at rise time is increased by an overdrive effect which is obtained by applying an extremely high voltage. In the meantime, the response used in an actual image display always corresponds to the response at decay time. Accordingly, the dependency on the tone level is extremely small. As a result, substantially equal response time is obtained throughout all the tones.
In light of the Formulae 1 to 3, conceivable measures for increasing the response speed of the nematic liquid crystal, which is the first technical trend, mainly include:    (1) reducing a cell gap and increasing electric field intensity while maintaining a constant voltage;    (2) increasing the electric field intensity by applying a high voltage to the liquid crystal to promote a state change of the liquid crystal (the overdrive method);    (3) increasing the dielectric anisotropy for sensitizing the response to the electric field;    (4) reducing the viscosity; and    (5) reducing the splay elastic constant (K11) and the bend elastic constant (K33) out of the elastic constants while increasing the twist elastic constant (K22) to speed up the response at decay time.
Generally speaking, the following problem arises when drive speed of twisted nematic liquid crystal is simply increased. When the drive speed of the nematic liquid crystal is increased, a capacity of the liquid crystal varies largely depending on the dielectric anisotropy and on a difference in alignment directions of the liquid crystal. For example, in case of liquid crystal known by the product name DLC-43002, the dielectric constant in the parallel direction is equal to 11.8 and the dielectric constant in the vertical direction is equal to 3.7. Accordingly, the capacity change is immense when the drive speed is increased and a holding voltage to be written in and held by a liquid crystal layer is thereby reduced. Such reduction in the holding voltage, or reduction in an effective applied voltage, causes deterioration in contrast because insufficient writing occurs and the liquid crystal is not shifted to a desired position. Moreover, when the same signal is repeated as in a still image, luminance continues to fluctuate until the holding voltage stops decreasing and several frames are required for obtaining the stable luminance.
To prevent the response which requires several frames, it is essential that a signal voltage to be applied and a transmittance to be obtained establish a one-on-one correlation. In the active matrix drive, the transmittance after the response of the liquid crystal is not determined by the applied signal voltage but by electric charges accumulated in a capacitor of the liquid crystal after the response thereof. This is because the active matrix drive adopts a drive mode based on constant electric charges, in which the liquid crystal makes a response by use of the accumulated electric charges.
The electric charges supplied from an active element are determined by the accumulated electric charges prior to writing a given signal and by newly written electric charges while disregarding small leaks and the like. Moreover, the accumulated electric charges after the response of the liquid crystal also vary depending on pixel design values such as physical constants of the liquid crystal, electric parameters, or storage capacitance.
For this reason, the following are required for establishing the proper correlation between the signal voltage and the transmittance, namely:    (1) a correlation between the signal voltage and the written electric charges;    (2) the accumulated electric charges prior to writing; and    (3) information required for calculation of the accumulated electric charges after the response, and actual calculation based on the information, and the like.As a result, it is necessary to install a frame memory for storing the values concerning (2) in terms of the entire screen, and a calculation unit for executing calculations concerning (1) and (3). Such installation invites an increase in the number of components in a system, and is therefore undesirable.
As a method of solving this problem, the reset pulse method configured to apply a reset voltage for resetting liquid crystal to a predetermined state before writing new data is often applied. As an example, the technique disclosed in International Display Research Conference (IDRC) 1997, pp. L-66 to L-99 will be described below.
The optically compensated birefringence (OCB) mode configured to align nematic liquid crystal into a pie-shape and to add a compensating film is applied in this document. The OCB mode is one of wide viewing angle modes. The OCB mode adopts a cell structure in which a phase compensating film (a biaxial retardation film) is provided to an antiparallel cell having a pretilt angle. A bias voltage is applied to homogeneous alignment to form bend alignment, and switching is performed by applying another voltage. The OCB mode has advantages of a wide viewing angle and short response time. The response speed of this liquid crystal mode ranges from approximately 2 ms to 5 ms, which is dramatically shorter than the conventional TN-TFT mode. Accordingly, the response is supposed to be completed within one frame. However, as described above, the holding voltage is considerably reduced by the variation in the dielectric constant attributable to the response of the liquid crystal, and several frames are required for obtaining the stable transmittance as similar to the conventional mode. Accordingly, there is proposed a method of writing a black display every time after writing a white display within one frame.
FIG. 1 is a graph showing variation in luminance according to the reset pulse method, in which the lateral axis indicates the time and the longitudinal axis indicates the luminance. In FIG. 1, a broken line indicates luminance variation in the case of normal drive, and a solid line indicates luminance variation when the reset pulse method is applied. As shown in FIG. 1, according to the normal drive method, the luminance is low at first two frames and reaches a stable level at the third frame. On the contrary, according to the reset pulse method, the luminance is always reset to a given state before writing new data and a one-on-one correlation equivalent to a certain written signal voltage to a certain transmittance is observed therein. Due to the one-on-one correlation, generation of driving signals is considerably simplified and means for storing the previous writing information such as a frame memory is not required therein.
Next, a configuration of a pixel in an active matrix liquid crystal display device will be described. FIG. 2 is a circuit diagram showing an example of a pixel circuit equivalent to one pixel in a conventional active matrix liquid crystal display device. As shown in FIG. 2, the pixel in the active matrix liquid crystal display device includes: an n-type metal oxide silicon (MOS) transistor (hereinafter referred to as an n-type transistor (Qn)) 904 of which a gate electrode is connected to a scan line 901, another one of electrodes is connected to a signal line 902, and the other electrode is connected to a pixel electrode 903; a storage capacitor 906 formed between the pixel electrode 903 and a storage capacitor electrode 905; and liquid crystal 908 interposed between the pixel electrode 903 and a counter electrode Vcom 907.
At present, liquid crystal display devices for laptop PCs and cellular telephones shaping the large application market for the liquid crystal display devices usually apply either an amorphous silicon thin film transistor (hereinafter referred to as an a-SiTFT) or a polysilicon thin film transistor (hereinafter referred to as a p-SiTFT) as the transistor (Qn) 904. Moreover, twisted nematic liquid crystal (hereinafter referred to as TN liquid crystal) is used as the liquid crystal material therein.
FIG. 3 is a circuit diagram showing an equivalent circuit of the TN liquid crystal. As shown in FIG. 3, the equivalent circuit of the TN liquid crystal can be expressed by a circuit in which a capacitance component C3 (its capacitance Cpix) of the liquid crystal, and, a value Rr of a resistance R1 as well as a capacitor C1 (its capacitance Cr), are connected in parallel. In this case, the resistance value Rr and the capacitance Cr are components which determine a response time constant of the liquid crystal.
FIG. 4 is a timing chart showing variations in a gate scan voltage Vg, a data signal voltage Vd, and a voltage at the pixel electrode 903 (hereinafter referred to as a pixel voltage) Vpix when the TN liquid crystal shown in FIG. 3 is driven by the pixel circuit shown in FIG. 2, in which the lateral axis indicates the time and the longitudinal axis indicates the voltage and the light transmittance. As shown in FIG. 4, the gate scan voltage Vg is set to a high level VgH in a period when this pixel is selected for horizontal scanning, whereby the n-type MOS transistor (Qn) 904 is set to on-state and the data signal voltage Vd inputted to the signal line 902 is transferred to the pixel electrode 903 through the n-type transistor (Qn) 904. The TN liquid crystal normally operates in the mode that allows light transmission when no voltage is applied thereto, or a so-called normally white mode.
At this time, a voltage for increasing the light transmittance of the light transmitted through the TN liquid crystal is applied for a period of several fields as the data signal voltage Vd. When the horizontal scanning period is over and the gate scan voltage Vg is set to a low level, the n-type transistor (Qn) 904 is set to off-state and the data signal voltage Vd transferred to the pixel electrode 903 is held by the storage capacitor 906 and by the capacitance Cpix of the liquid crystal. In this case, the pixel voltage Vpix causes voltage shifts called feedthrough voltages through source-gate capacitance of the n-type transistor (Qn) 904 at the time when the n-type transistor (Qn) 904 is set to off-state. The voltage shifts are indicated by Vf1, Vf2, and Vf3 in FIG. 4, and amounts of the voltage shifts Vf1 to Vf3 can be reduced by designing a large value for the storage capacitor 906.
The pixel voltage vpix is held in the subsequent field period until the gate scan voltage Vg is set to the high level again and the transistor (Qn) 904 is selected. In response to the pixel voltage Vpix thus held, the TN liquid crystal performs switching and the light transmitted through the liquid crystal transits from a dark state to a bright state as indicated by light transmittance T1. At this time, as shown in FIG. 4, the pixel voltage Vpix is shifted only by ΔV1, ΔV2, and Δ3 in the respective fields during the holding period. Such an aspect is attributable to variation in the capacitance of the liquid crystal which occurs in accordance with the response of the liquid crystal. To reduce such variation, the size of the storage capacitor 906 is normally designed at least two or three times larger than the pixel capacitance Cpix. In this way, it is possible to drive the TN liquid crystal by use of the pixel circuit shown in FIG. 2.
Meanwhile, as a technique having combined effects of the overdrive method and the reset method, Japanese Patent Publication No. 2001-506376 discloses a technique to modulate a common voltage which is a voltage at a common electrode (such as a counter electrode) disposed opposite to a pixel electrode. This technique will be described with reference to FIG. 5. FIG. 5 is a graph showing an operation of the technique to modulate the common voltage, in which the lateral axis indicates the time and the longitudinal axis indicates the voltage and the current.
The common voltage has been conventionally driven at a constant value for a period of one frame cycle (a period from t0 to t2 (or a period from t2 to t4) in FIG. 5 will be defined as one frame cycle), or alternatively, subjected to common inversion drive where the one frame cycle was further divided into two sub-periods and the voltage value was inverted between these two sub-periods. On the contrary, in the technique to modulate the common voltage, the common voltage which is the voltage at the common electrode disposed opposite to the pixel electrode is modulated as shown in FIG. 5. An upper part of FIG. 5 shows variation of the common voltage (VCG) with time and a lower part thereof shows variation of the light transmittance (I) with time which is caused by the response of the liquid crystal. Specifically, a voltage waveform 151 represents a voltage waveform to be applied to the common electrode, an optical intensity waveform 152 represents an optical intensity waveform in a time scale corresponding to the waveform 151, and lines 153 to 156 represent pixel optical intensity curves.
One frame cycle is divided into two sub-periods and a voltage having approximately the same amplitude as in the conventional common inversion drive is applied for a sub-period from t1 to t2 (or a sub-period from t3 to t4). On the contrary, a voltage having higher amplitude than the amplitude in the common inversion drive (such as a voltage higher than the amplitude in the common inversion drive by an amount equivalent to performing a black display) is applied for a sub-period from t0 to t1 (or a sub-period from t2 to t3). According to this technique, it is possible to change the entire display area into the black display at high speed by an effect of an increased voltage differential between the pixel electrode and the common electrode in the sub-period from t0 to t1 when the high voltage is applied to the common electrode. In other words, a driving operation corresponding to the reset drive takes place. In addition, even when image data are written in the pixel electrode in the sub-period from t0 to t1, such data are not observed as a display image because a potential difference from the common electrode is sufficiently large (by an amount of the voltage for the black display, for example). The voltage at the common electrode is set back to the amplitude for the common inversion at the timing of t1 after completion of writing the image data in the entire display area. As a consequence, the liquid crystal layer starts the response equivalent to the transmittance corresponding to each tone level in accordance with the voltage memorized in the pixel electrode. In other words, the voltage differential always changes from the high level to the level corresponding to the voltage value that represents each tone when starting the response. In this context, a sort of overdrive operation takes place in the sub-period from t0 to t1.
As a method of improving the response speed of the TN liquid crystal, there is also a method of increasing the speed by applying a different display mode using nematic liquid crystal. The method includes a method utilizing the electrically controlled birefringence (ECB) mode by taking advantage of birefringence, and a method utilizing the above-described optically compensated birefringence (OCB) mode, for example.
In addition, as the second trend, i.e. as the method of using liquid crystal other than the twisted nematic liquid crystal which can achieve high speed response, there is a technique to use spontaneous polarization type smectic liquid crystal.
However, the above-described conventional techniques have the following problems. First of all, as described previously, the conceivable measures for improving the response speed of the twisted nematic liquid crystal include (1) the measure to reduce the cell gap, (2) the measure to apply the high voltage to the liquid crystal, (3) the measure to increase the dielectric anisotropy, (4) the measure to reduce the viscosity, and (5) the measure to reduce the splay and bend elastic constants and to increase the twist elastic constant.
Of these measures, the cell gap (the thickness of the liquid crystal layer) in the measure (1) can be changed only within a certain relation with refractive index anisotropy Δn so as to obtain a sufficient optical effect. In the meantime, the viscosity in the measure (2), the dielectric anisotropy in the measure (3), and the elastic constants in the measure (5) are all physical values. Accordingly, the values depend largely on the nature of materials and it is therefore extremely difficult to change only the physical values of each material. As a consequence, it is hard to achieve high speed effects as estimated from the formulae. For example, whereas K11, K22, and K33 are mutually independent elastic constants, a correlation defined as K11:K22:K33=10:5:14 is satisfied in many cases according to results of measurement of actual materials (oral disclosure by Merck). Therefore, it is not always appropriate to treat these constants as the independent constants. For instance, it is possible to derive a formula K=11×K22/5 by use of the above-mentioned correlation and the Formula 3. In this case, only K22 is deemed independent.
Meanwhile, the applied voltage in the measure (4) is also restricted because an increase in the voltage causes an increase in power consumption as well as a cost increase in a drive circuit. Moreover, when active elements such as thin film transistors are provided and driven in a display device, the applied voltage is also restricted by withstanding voltage of the elements. Accordingly, the conventional efforts to increase the response speed are almost marginal.
In the meantime, the above-described reset pulse method disclosed in the IDRC 1997 pp. L-66 to L-69 can improve the response speed of the TN type display device to some extent. However, as shown in FIG. 1, the reset pulse method can only achieve a display screen which hardly realizes the original degree of luminance the device is designed for. As a result, there still remains the problem of low light use efficiency and a failure to acquire sufficient light transmittance.
The reason is the fact that the response speed of the typical nematic liquid crystal is slow, and that the response speed of the most widely used TN liquid crystal is slow in particular. Due to the slow response speed, the display device cannot rise sufficiently from the black display at the reset state to the image display mode within a required period, and thereby fails to obtain the required light transmittance. That is, when switching from the black display into the white display, the display device fails to achieve the complete white display and ends up with gray display. On the other hand, when switching from the white display to the black display, the display device fails to achieve the complete black display and ends up with the gray display similarly. Moreover, in a tone display, the response speed may be even slower as described previously. Accordingly, the device cannot achieve expected tone levels when displaying motion images or the like.
In addition, in this reset method the slow response speed also causes the following problem. For example, there is a case where the liquid crystal does not respond sufficiently to achieve the complete black display upon resetting. In this case, the same transmittance may not be obtained by writing the same piece of data for several times. Such an aspect is attributable to a failure to establish the complete predetermined state of alignment of the liquid crystal as a reset operation is imperfect. In this case, the liquid crystal exhibits the transmittance as the response after resetting which reflects a hysteresis of a previous frame. As a consequence, the one-on-one correlation between the applied voltage and the transmittance is not satisfied any longer.
Furthermore, there are problems concerning a slow start of optical response of the liquid crystal after resetting and observation of abnormal optical response prior to the start of the normal optical response. The reason is that the direction of action of the liquid crystal is not definite at the point when the liquid crystal transits from the predetermined state of alignment achieved by resetting to the normal response, and the liquid crystal may perform uneven or unstable response instead.
It is considered that the response time of the TN liquid crystal is equivalent to several tens of milliseconds as a sum of the response time for rising and the response time for decaying, and that the response time becomes much longer and would reach a hundred milliseconds or more as the response during the tone display. As shown in the Formula 1 described above, the response time for rising is normally proportional to a squared value of the thickness. Therefore, a transmissive display device requires four times more response time than a reflective display device. As a result, light use efficiency is significantly deteriorated in the transmissive display device.
Meanwhile, the method of driving the TN liquid crystal in the OCB mode or the like has a problem of stringent demands for production accuracy and uniformity, and a resultant decline in yields. This is because the thickness of the liquid crystal layer requires higher accuracy, or because it is necessary to use optical elements such as compensating films or retardation films that satisfy a high degree of uniformity.
For example, in the case of the electrically controlled birefringence (ECB) mode utilizing the birefringence, the demand for the accuracy of the thickness of the liquid crystal layer is about 3% of the thickness of the liquid crystal layer in order to achieve the contrast equal to 100. In that case, when the thickness is equal to or less than 3 μm, the unevenness in the thickness must be equal to or less than 90 nm. When higher display conditions such as the contrast are required, the demand for the accuracy of the thickness will be even more stringent.
Similarly, the OCB mode also has problems that it is generally difficult to obtain the uniform and stable bend alignment, and moreover, that a high degree of stability is required in the manufacturing process because the cell gap (the thickness of the liquid crystal) and properties of compensating films must be highly uniform.
In either case, the demand for the accuracy of the thickness of the liquid crystal layer is about 3% of the thickness of the liquid crystal layer in order to achieve the contrast equal to 100. In that case, when the thickness is equal to or less than 3 μm, the unevenness in the thickness must be equal to or less than 90 nm. When higher display conditions such as the contrast are required, the demand for the accuracy of the thickness will be even more stringent.
Furthermore, the technique to use the spontaneous polarization type smectic liquid crystal, which can achieve high-speed response, also has the following problems. Specifically, the spontaneous polarization type smectic liquid crystal is also associated with stringent demands for production accuracy and uniformity, and with a resultant decline in yields. This is because, when a smectic liquid crystal mode is used instead of the TN mode in order to increase the response speed, the thickness of the liquid crystal layer requires a higher accuracy. Another reason is that it is necessary to use optical elements such as compensating films or retardation films that satisfy a high degree of uniformity. In the meantime, the spontaneous polarization type smectic liquid crystal requires an extremely high level of flatness of a substrate surface as compared to the nematic liquid crystal of the TN type and the like. In this case, even tiny irregularities on a surface of an indium tin oxide (ITO) electrode, which is a transparent electrode, become problematic.
In this way, all the above-described conventional methods have difficulties in further improving the response speed while maintaining practical yields.